Detection of liquid in gas pipeline

ABSTRACT

An apparatus and related method for measuring the presence or degree of stratified flow in a two-phase flow is disclosed. A first speed of sound for the fluid flowing through the pipeline is measured for an ultrasonic signal that would reflect from stratified flow, if present. A second speed of sound is measured at a location that would not reflect off the stratified flow. A difference in these two measurements indicates the presence of stratified flow. The level of stratified flow can be determined based on the magnitude of the difference. Because this method is so sensitive to changes in the amount of stratified flow, it is more reliable than previously known methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part and claims benefit topending U.S. patent application Ser. No. 09/388,253 filed Sep. 1, 1999entitled “Ultrasonic 2-Phase Flow Apparatus and Stratified LevelDetector.”

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The invention relates generally to the detection of liquid in apipeline. More particularly, embodiments of the invention relate to thedetection of stratified flow in a pipeline. An embodiment of theinvention detects the presence and volume of stratified flow in apipeline based on time of flight or velocity of sound measurements foran ultrasonic meter.

[0005] 2. Description of the Related Art

[0006] After a hydrocarbon such as natural gas has been removed from theground, it is commonly transported from place to place via pipelines.Often this gas stream also contains a certain amount, or percentfraction, of liquid. As is appreciated by those of skill in the art, itis desirable to know with accuracy the amount of gas in the gas stream.It is also extremely desirable to know whether liquid is beingtransported along with the gas stream. For example, the presence of astratified flow of liquid in the gas stream may indicate a productionproblem upstream of the measurement device. A “stratified flow” ofliquid consists of a stream or river traveling along one area of thepipeline, such as the bottom. If the measurement device is at thelocation where the gas is changing hands or custody, and if the gascontains “natural gas liquids” or condensates, a seller of gas wantsextra compensation for this energy-rich liquid.

[0007] Gas flow meters have been developed to determine how much gas isflowing through the pipeline. One type of meter to measure gas flow iscalled an ultrasonic flow meter. Ultrasonic flow meters are also namedsonic or acoustic flow meters.

[0008]FIG. 1A shows an ultrasonic meter suitable for measuring gas flow.Spoolpiece 100, suitable for placement between sections of gas pipeline,has a predetermined size and thus defines a measurement section. A pairof transducers 120 and 130, and their respective housings 125 and 135,are located along the length of spoolpiece 100. A path 110, sometimesreferred to as a “chord” exists between transducers 120 and 130 at anangle θ to a centerline 105. The position of transducers 120 and 130 maybe defined by this angle, or may be defined by a first length L measuredbetween transducers 120 and 130, a second length X corresponding to theaxial distance between points 140 and 145, and a third length Dcorresponding to the pipe diameter. Distances X, D and L are preciselydetermined during meter fabrication. Points 140 and 145 define thelocations where acoustic signals generated by transducers 120 and 130enter and leave gas flowing through the spoolpiece 100 (i.e. theentrance to the spoolpiece bore). In most instances, meter transducerssuch as 120 and 130 are placed a specific distance from points 140 and145, respectively, regardless of meter size (i.e. spoolpiece size). Afluid, typically natural gas, flows in a direction 150 with a velocityprofile 152. Velocity vectors 153-158 indicate that the gas velocitythrough spool piece 100 increases as centerline 105 of spoolpiece 100 isapproached.

[0009] Transducers 120 and 130 are ultrasonic transceivers, meaning thatthey both generate and receive ultrasonic signals. “Ultrasonic” in thiscontext refers to frequencies above about 20 kilohertz. Typically, thesesignals are generated and received by a piezoelectric element in eachtransducer. Initially, D (downstream) transducer 120 generates anultrasonic signal that is then received at, and detected by, U(upstream) transducer 130. Some time later, U transducer 130 generates areciprocal ultrasonic signal that is subsequently received at anddetected by D transducer 120. Thus, U and D transducers 130 and 120 play“pitch and catch” with ultrasonic signals 115 along chordal path 110.During operation, this sequence may occur thousands of times per minute.

[0010] The transit time of the ultrasonic wave 115 between transducers U130 and D 120 depends in part upon whether the ultrasonic signal 115 istraveling upstream or downstream with respect to the flowing gas. Thetransit time for an ultrasonic signal traveling downstream (i.e. in thesame direction as the flow) is less than its transit time when travelingupstream (i.e. against the flow). The upstream and downstream transittimes can be used to calculate the average velocity along the signalpath. In particular, the transit time t₁, of an ultrasonic signaltraveling against the fluid flow and the transit time t₂ of anultrasonic signal travelling with the fluid flow may be defined:$\begin{matrix}{t_{1} = \frac{L}{c - {V\frac{x}{L}}}} & (1) \\{t_{2} = \frac{L}{c + {V\frac{x}{L}}}} & (2)\end{matrix}$

[0011] where,

[0012] c=speed of sound in the fluid flow;

[0013] V=average axial velocity of the fluid flow over the chordal pathin the axial direction;

[0014] L=acoustic path length;

[0015] x=axial component of L within the meter bore;

[0016] t₁=transmit time of the ultrasonic signal against the fluid flow;and

[0017] t₂=transit time of the ultrasonic signal with the fluid flow.

[0018] The upstream and downstream transit times can be used tocalculate the average velocity along the signal path by the equation:$\begin{matrix}{V = {\frac{L^{2}}{{2x}\quad}\frac{t_{1} - t_{2}}{t_{1}t_{2}}}} & (3)\end{matrix}$

[0019] with the variables being defined as above.

[0020] The upstream and downstream travel times may also be used tocalculate the speed of sound in the fluid flow according to theequation: $\begin{matrix}{c = {\frac{L\quad}{2\quad}\frac{t_{1} + t_{2}}{t_{1}t_{2}}}} & (4)\end{matrix}$

[0021] Given the cross-section measurements of the meter carrying thegas, the average velocity over the area of the gas may be used to findthe quantity of gas flowing through spoolpiece 100. Typically, thesemeasurements are based on a batch of 10-30 ultrasonic signals ratherthan upon only one upstream and downstream signal. Alternately, a metermay be designed to attach to a pipeline section by, for example, hottapping, so that the pipeline dimensions instead of spoolpiecedimensions are used to determine the average velocity of the flowinggas.

[0022] In addition, ultrasonic gas flow meters can have one or morepaths. Single-path meters typically include a pair of transducers thatprojects ultrasonic waves over a single path across the axis (i.e.center) of spoolpiece 100. In addition to the advantages provided bysingle-path ultrasonic meters, ultrasonic meters having more than onepath have other advantages. These advantages make multi-path ultrasonicmeters desirable for custody transfer applications where accuracy andreliability are crucial.

[0023] Referring now to FIG. 1B, a multi-path ultrasonic meter is shown.Spool piece 100 includes four chordal paths A, B, C, and D at varyinglevels through the gas flow. Each chordal path A-D corresponds to twotransceivers behaving alternately as transmitter and receiver. Alsoshown is an electronics module 160, which acquires and processes thedata from the four chordal paths A-D. This arrangement is described inU.S. Pat. No. 4,646,575, the teachings of which are hereby incorporatedby reference. Hidden from view in FIG. 1B are the four pairs oftransducers that correspond to chordal paths A-D.

[0024] The precise arrangement of the four pairs of transducers may bemore easily understood by reference to FIG. 1C. Four pairs of transducerports are mounted on spool piece 100. Each of these pairs of transducerports corresponds to a single chordal path of FIG. 1B. A first pair oftransducer ports 125 and 135 including transducers 120 and 130 ismounted at a non-perpendicular angle θ to centerline 105 of spool piece100. Another pair of transducer ports 165 and 175 including associatedtransducers is mounted so that its chordal path loosely forms an “X”with respect to the chordal path of transducer ports 125 and 135.Similarly, transducer ports 185 and 195 are placed parallel totransducer ports 165 and 175 but at a different “level” (i.e. adifferent radial position in the pipe or meter spoolpiece). Notexplicitly shown in FIG. 1C is a fourth pair of transducers andtransducer ports. Taking FIGS. 1B and 1C together, the pairs oftransducers are arranged such that the upper two pairs of transducerscorresponding to chords A and B form an X and the lower two pairs oftransducers corresponding to chords C and D also form an X.

[0025] Referring now to FIG. 1B, the flow velocity of the gas may bedetermined at each chord A-D to obtain chordal flow velocities. Toobtain an average flow velocity over the entire pipe, the chordal flowvelocities are multiplied by a set of predetermined constants. Suchconstants are well known and were determined theoretically.

[0026] This four-path configuration has been found to be highly accurateand cost effective. Nonetheless, other ultrasonic meter designs areknown. For example, other ultrasonic meters employ reflective chordalpaths, also known as “bounce” paths.

[0027] A pipeline may carry liquid in addition to the gas stream. Liquidlevel detectors are known that detect whether liquid is present at alocation of interest, although typically these liquid level detectorsare not positioned inside a pipeline.

[0028] One known design of ultrasonic meter is disclosed in U.S. Pat.No. 5,719,329 to Jepson. In particular, this patent discloses amultiphase flow meter that measures film heights of the fluids flowingthough the meter by a very complicated scheme using the densities of themediums in the meter, the pressure of the transmitted wave to theincident wave, and the velocity of sound in the medium. Unfortunately,this multiphase meter is likely too complicated for use in real worldapplications. The disclosure of the patent also includes a method toconfirm the calculated film height by reflection of an ultrasonic signalfrom, and back to, a single transducer located on the bottom of themeter bore. Unfortunately, however, this method to determine film levelis not very accurate (which may be why it is used only as a confirmationand not by itself).

[0029] Therefore, a meter or device is needed that is capable ofdetecting liquid in a pipeline. This device might also measure theamount of stratified flow in a gas stream. The device would be bothsimple and accurate enough to be used in real-world applications.

SUMMARY OF THE INVENTION

[0030] Disclosed embodiments of the invention include a method todetermine the level of stratified flow in a conduit such as a pipeline,including transmitting an ultrasonic signal through a medium from afirst transducer, reflecting the ultrasonic signal from the surface ofthe stratified flow, receiving the ultrasonic signal at a secondtransducer, and computing the speed of sound for the ultrasonic signalthrough the medium (the speed of sound may also be based on a batch ofmeasurements along this same chord). A second speed of sound is alsocomputed based on other ultrasonic signals. A difference in these twocomputed speeds of sound indicates the presence of stratified flow.Analysis of the difference in these two speeds of sound indicates thelevel of stratified flow in the pipeline.

[0031] The present invention comprises a combination of features andadvantages that enable it to overcome various problems of prior devices.The various characteristics described above, as well as other features,will be readily apparent to those skilled in the art upon reading thefollowing detailed description of the preferred embodiments of theinvention, and by referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] For a more detailed description of the preferred embodiment ofthe present invention, reference will now be made to the accompanyingdrawings, wherein:

[0033]FIG. 1A is a cut-away top view of an ultrasonic gas flow meter;

[0034]FIG. 1B is an end view of a spoolpiece including chordal pathsA-D;

[0035]FIG. 1C is a top view of a spoolpiece housing transducer pairs;

[0036]FIG. 2 is an end view of a multi-transducer leveldetector/ultrasonic flow meter combination;

[0037]FIG. 3 is a top view of a pipeline showing flight paths for chordsA, B, and C;

[0038]FIG. 4 is a side view of path D in a pipeline with no stratifiedflow;

[0039]FIG. 5 is a side view of a pipeline having stratified flowcontrasting the flight paths of path D;

[0040]FIG. 6 is a graph illustrating changes in speed of sound versuschanges in stratified flow depth;

[0041]FIG. 7 is a graph illustrating changes in speed of sound versuschanges in area for the stratified flow;

[0042]FIG. 8 is an end view of a two-chord ultrasonic meter; and

[0043]FIG. 9 is a top view of a horizontal chord.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0044]FIG. 2 includes the end-view of a pipeline or spoolpiece 2000 fora multiple transducer level detector. As used herein, the term pipelineshall refer to either an actual pipeline or to a spoolpiece. Threechords 2010, 2020, 2030 (corresponding to a multi-path ultrasonic meter)are shown and are labeled as chords A, B, and C. Path D, labeled 2040,is also shown and corresponds to an additional, vertical chord. Ofcourse, path D includes an upstream transducer and a downstreamtransducer. The transducers corresponding to path D may be positionedsomewhere other than true vertical, so long as the length of path Dchanges with changes in stratified fluid. Chord D therefore may be thefourth chord of a four-chord ultrasonic meter, or may be used separatelybut in conjunction with an ultrasonic flow meter (or other device tomeasure the speed of sound in the gas) to establish the amount ofstratified flow. Use of a four-chord ultrasonic meter is exemplary onlyand the disclosed multiple transducer level detector could be used withor be part of any multiple chord ultrasonic meter, including a meterhaving bounce paths. Nonetheless, use of a three horizontal chord designin conjunction with the disclosed level detector (or integrating thedesign into a four-chord meter) has certain advantages over the use of atraditional four horizontal-chord design. For example, the lowest chordon the 4 chordal path meter is easily flooded in stratified flow. Anultrasonic meter having three horizontal chords does not have thislowest chord, and thus avoids this problem. As already stated, theultrasonic transducers corresponding to path D can be either separatefrom, or as a part of, such an ultrasonic meter. For these purposes, alevel detector and an ultrasonic meter are thought of as two differentdevices that operate together to achieve a synergistic effect, but inactuality these two devices may equally be part of the same device andshare components such as electronics, etc.

[0045]FIG. 3 shows the pipeline 2000 from a top perspective andidentifies a direction of flow, as well as chords A, B, and C. FIG. 4includes a side view of pipeline 2000 when the pipeline does not containa stratified 2-phase flow. Path D originates at point 2210 correspondingto an ultrasonic transducer 2215, reflects off of the bottom 2230 ofpipeline 2000 and travels to point 2220 corresponding to ultrasonictransducer 2225. The transducers 2215 and 2225 are preferably angled atabout 60 degrees, although this is not a requirement of the invention.During operation transducers 2215 and 2225 preferably each generateultrasonic signals that travel along path D and are detected by theother transducer, resulting in both an upstream and a downstreammeasurement. The measurement of both the upstream and downstream timesof flight yields a speed of sound measurement for chord D.

[0046]FIG. 5 shows a side view of a pipeline 2000 containing astratified flow 2310 of depth “h”. In a pipeline, the area of thepipeline occupied by stratified flow will typically not exceed 10%.Pipeline 2000 includes ultrasonic transducers 2215 and 2225 thatgenerate ultrasonic signals that travel along a first path D 2041. FirstD path 2041 corresponds to a pipeline without stratified flow as shownin FIG. 6. Second path D 2042, corresponding to a pipeline with astratified flow, also is shown. Second path D 2042 corresponds to anultrasonic signal that reflects off the surface 2320 of stratified flow2310. In addition, it can be seen that second path D 2042 is slightlyshorter than first path D 2041. In particular, the second path D 2042will be slightly shorter than the first path D 2041 depending upon thelevel of the stratified flow.

[0047] This variation in path length is used by the invention toestablish the presence of stratified flow. A more complicated analysisallows the calculation of the level of the stratified flow. Either way,because these level variations are often slight, it is preferable toutilize a measurement very sensitive to these changes in path length.Once the measurements indicate that stratified flow is present, thelevel, area, flow amount, etc., of the stratified flow can be found.

[0048] It has been found that the level of the stratified flow shouldresult in a significant effect on a speed of sound measurement alongchord D. Unlike the typical level detector of the prior art, in a gaspipeline the pressure, temperature, and gas composition are variable.This complicates the determination of whether there exists liquid in thebottom of the conduit being measured. This variation in pressure,temperature and gas composition also makes the speed of sound generallyunknown. Thus, if speed of sound is the parameter used to determine thepresence of level of liquid in a pipeline, as it is in the preferredembodiment of the present invention, the speed of sound through theflowing gas should be normally be measured independently and nearsimultaneously with the detection of level in the pipeline. Suchcontemporaneous measurement of the speed of sound should be made closeenough in time to the first that there can be a high level of assurancethat variables such as temperature, pressure, and composition have notchanged enough to signficantly affect the measurement of stratifiedflow. Where the speed of sound is being measured by an ultrasonic meter,this measurement is made in addition to measuring the velocity of thegas flow along the chords of the ultrasonic meter (such as an ultrasonicmeter having chords 2010, 2020, 2030). As an additional advantage to theinvention, speed of sound is a measurement that is already made byultrasonic meters, and thus for the preferred embodiment, minimalchanges are required to existing meters.

[0049] When there is no liquid in the bottom of the pipeline, the speedof sound measured along path D will be the same as the speed of soundmeasured from the other chords in the ultrasonic meter. With liquid inthe bottom of the pipe, however, the reflection is from the liquidsurface (not the bottom of the pipe) and path D is shorter. This makesthe transit time shorter and the calculated speed of sound (using thewrong path length) is higher. The difference between the speed of soundmeasured by chords A, B, and C and the speed of sound measured by chordD can be used to establish the level of stratified flow in pipeline2000.

[0050] Referring to FIG. 6, calculations show the effect of h/D (withh=depth of stratified flow and D=pipe diameter) on the change in speedof sound (Delta VOS) and the area occupied by the liquid as a percent ofthe total pipe area. The sensitivity of speed of sound to changes in thearea occupied by the stratified flow can be seen in FIG. 7. As can beseen, a 5% change in area is accompanied by an 8% change in the speed ofsound, giving a very sensitive measurement.

[0051] As will now be apparent to one of ordinary skill in the art,there is no requirement that a four-chord ultrasonic meter be used withthese principles to determine the level of stratified flow in thepipeline. A two-chord level detector as shown in FIG. 8 could also beemployed.

[0052]FIG. 8 illustrates an end view of a spoolpiece with chord A andchord B. Ideally, these two chords are vertical and horizontal.Therefore, chord A will loosely be referred to as a vertical chord,while chord B will loosely be referred to as a horizontal chord. Chord Bincludes two transducers, an upstream transducer and a downstreamtransducer. Chord A includes two transducers, similar to that shown inFIGS. 4 and 5. As explained above, a difference in upstream anddownstream travel times for each of these chords provides (among otherthings) a velocity of sound (VOS) measurement for any fluid the chord istravelling through.

[0053] With no appreciable liquid on the bottom of the pipeline, thevelocity of sound measured on the horizontal and vertical chords is thesame. With liquid in the bottom of the pipe, the ultrasonic pulse isreflected from the liquid surface and not the pipe wall. This creates ashorter path length and a shorted transit time is measured. Because anycomputer or processor associated with the meter bases its VOScalculations on an assumed path length, this results in a highermeasured velocity of sound. The difference in VOS between the verticaland horizontal chord, is related by geometry to the liquid level andarea occupied by the liquid, as shown in FIGS. 6 and 7.

[0054] The speed of the stratified flow can be calculated according tothe square root of the ratio of the densities of the flows (i.e. gaseousand stratified) times the speed of the gas. This relationship can bederived because in equilibrium the shear stress of the liquid on thebottom of the pipe is the same as the shear stress of the gas on theliquid. In other words, equilibrium is achieved when the friction at thesurface of the stratified flow is the same as the friction at the bottomof the pipe. Consequently,

ρ_(L) V _(L) ²=ρ_(G) [V _(G) ^(−V) _(L)]²  (9)

[0055] Where,

[0056] ρ_(L)=density of the liquid

[0057] ρ_(G)=density of the gas

[0058] V_(L)=velocity of liquid

[0059] V_(G)=velocity of gas

[0060] This means that, $\begin{matrix}{V_{L} = \frac{V_{G}}{1 + \sqrt{\frac{\rho_{L}}{\rho_{G}}}}} & (10)\end{matrix}$

[0061] Upon understanding of the teachings above, a processor orcomputer may be programmed to detect the presence of liquid in thepipeline by a statistically significant discrepancy between the measuredspeeds of sound. The processor or computer may be programmed to estimateaccurately the level of liquid in the pipeline. The processor orcomputer may also be programmed to determine not only the level ofliquid in the pipeline, but also the speed of the flow. Such processorsmay be part of an ultrasonic meter or may be separate. For example thisprocessor may be the same microprocessor that operates on measured datafrom an ultrasonic meter.

[0062] While preferred embodiments of this invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit or teaching of this invention. Theembodiments described herein are exemplary only and are not limiting.Many variations and modifications of the system and apparatus arepossible and are within the scope of the invention. Accordingly, thescope of protection is not limited to the embodiments described herein,but is only limited by the claims that follow, the scope of which shallinclude all equivalents of the subject matter of the claims.

What is claimed is:
 1. A method to determine the presence of stratified2-phase flow through a conduit, comprising: (a) transmitting anultrasonic signal through a first medium at a first time from a firstultrasonic transducer; (b) reflecting said ultrasonic signal from thesurface of a stratified flow of liquid through said conduit; (c)receiving said ultrasonic signal at a second ultrasonic transducersecond time; (d) computing a first speed of sound for said first mediumbased at least in part on said first time and said second time; (e)determining a second speed of sound for said first medium not based onsaid first ultrasonic signal; and (f) determining whether stratifiedflow exists in said conduit by comparing said first speed of sound tosaid second speed of sound.
 2. The method of claim 1, wherein saidstratified flow is on the bottom of a pipeline and said first and secondtransducers are at the top of said pipeline.
 3. The method of claim 1,wherein said second speed of sound is determined contemporaneously withsaid first speed of sound.
 4. The method of claim 3, wherein saidreceiving is second speed of sound is determined by third and fourthultrasonic transducers.
 5. The method of claim 1, wherein said receivingis second speed of sound is determined by third and fourth ultrasonictransducers.
 6. The method of claim 5, wherein said third and fourthultrasonic transducers form a horizontal chord for travel of said secondultrasonic signal.
 7. A method to determine the level of stratified2-phase flow through a conduit, comprising: (a) transmitting anultrasonic signal through a first medium at a first time from a firstultrasonic transducer; (b) reflecting said ultrasonic signal from thesurface of a stratified flow of liquid through said conduit; (c)receiving said ultrasonic signal at a second ultrasonic transducersecond time; (d) computing a first speed of sound for said first mediumbased at least in part on said first time and said second time; (e)determining a second speed of sound for said first medium not based onsaid first ultrasonic signal; and (f) deriving the level of stratifiedflow in said conduit by comparing said first speed of sound to saidsecond speed of sound.
 8. The method of claim 7, where said stratifiedflow is on the bottom of a pipeline and said first and secondtransducers are at the top of said pipeline.
 9. The method of claim 7,wherein said second speed of sound is determined contemporaneously withsaid first speed of sound.
 10. The method of claim 9, wherein saidreceiving is second speed of sound is determined by third and fourthultrasonic transducers.
 11. The method of claim 7, wherein saidreceiving is second speed of sound is determined by third and fourthultrasonic transducers.
 12. The method of claim 11, wherein said thirdand fourth ultrasonic transducers form a horizontal chord for travel ofsaid second ultrasonic signal.
 13. The method of claim 11, wherein saidlevel of said stratified flow is determined based on the difference insaid first speed of sound and said second speed of sound.
 14. A leveldetector in a pipeline to measure stratified flow of less than 10% insaid pipeline, comprising: at least four ultrasonic transducers attachedto said pipeline and positioned to reflect a first set of ultrasonicsignals from the surface of said stratified flow and positioned to avoidreflecting ultrasonic signals from a surface of stratified flow; aprocessor associated with said at least four ultrasonic transducers tocalculate the presence of stratified flow in said pipeline based on afirst speed of sound and a second speed of sound, wherein said firstspeed of sound is measured based on said first set of signals and saidsecond speed of sound is measured based on said second set of signals.15. The level detector of claim 14, wherein a difference between saidmeasured speed of sound and said second measured speed of sound providesan indication of said level of said stratified flow.
 16. The leveldetector of claim 14, wherein said stratified flow is on the bottom of apipeline and said at least two of said four transducers are at the topof said pipeline.
 17. A method to determine the amount of stratifiedflow through a conduit, comprising: (a) transmitting through a firstportion of said conduit a first ultrasonic signal from a first upstreamlocation; (b) receiving said first ultrasonic signal at a locationdownstream of said first upstream location; (c) transmitting throughsaid first portion a second ultrasonic signal from a first downstreamlocation; (d) receiving said second ultrasonic signal at a locationupstream of said first downstream location; (e) transmitting throughsaid first portion of said conduit a third ultrasonic signal from asecond upstream location, said third ultrasonic signal reflecting of asurface of said stratified flow; (f) receiving said third ultrasonicsignal at a location downstream of said second upstream location; (g)transmitting through said first portion a fourth ultrasonic signal froma second downstream location, said fourth ultrasonic signal reflectingof said surface of said stratified flow; (h) receiving said fourthultrasonic signal at a location upstream of said second downstreamlocation; (i) computing the amount of said stratified flow in saidconduit based on the travel times of said first, second, third, andfourth ultrasonic signals.
 18. The method of claim 17, wherein saidfirst portion is not said stratified flow.
 19. The method of claim 17,wherein said first portion is a gas.
 20. The method of claim 17, whereinsaid first and second ultrasonic signals travel in a generallyhorizontal direction.
 21. The method of claim 20, wherein said first andsecond ultrasonic signals are used to measure a speed of sound for aportion of said conduit not carrying said stratified flow.
 22. Themethod of claim 17, wherein said first and second ultrasonic signalstravel in generally horizontal directions and said third and fourthultrasonic signals travel in generally vertical directions.
 23. Themethod of claim 22, wherein said first and second ultrasonic signals areused to measure a speed of sound for a portion of said conduit notcarrying said stratified flow and said third and fourth ultrasonicsignals are used to measure a second speed of sound corresponding to alevel of said stratified flow in said conduit.
 24. The method of claim17, wherein said first ultrasonic signal is transmitted by a firsttransducer and received by a second transducer, said second ultrasonicsignal is transmitted by said second transducer and received by saidfirst transducer, said third ultrasonic signal is transmitted by a thirdtransducer and received by a fourth transducer, and said fourthultrasonic signal is transmitted by said fourth transducer and receivedby said third transducer.
 25. The method of claim 17, wherein said stepof computing includes calculating a first measured speed of sound fromsaid first and second ultrasonic signals, and a second measured speed ofsound based on said third and fourth ultrasonic signals, the discrepancybetween said first and second measured speeds of sound indicating thelevel of said stratified flow.
 26. The method of claim 17, wherein saidmethod is performed by a two-chord ultrasonic meter.
 27. A flow metersuitable to determine the level of stratified flow through a conduit,comprising: a first transducer suitable to transmit a first ultrasonicsignal across said conduit and through a first medium traveling throughsaid first medium from an upstream end to a downstream end; a secondtransducer suitable to receive said first ultrasonic signal and totransmit to said first transducer a second ultrasonic signal; a thirdtransducer suitable to transmit a third ultrasonic signal through saidfirst medium, said third ultrasonic signal positioned to reflect from asurface of said stratified flow; and a processor to compute an upstreamtransit time for said first signal, a downstream time for said secondsignal, and a level reflection transit time for said third ultrasonicsignal, said processor further computing a level of stratified flowbased upon said upstream transit time, said downstream transit time, andsaid level detection transit time.
 28. The flow meter of claim 27,further comprising: a fourth transducer suitable to receive said thirdultrasonic signal and to transmit to said third transducer a secondlevel reflection transit time, wherein said processor additionally usessaid second level reflection transit time to compute said level ofstratified flow.
 29. The flow meter of claim 28, wherein a first speedof sound is computed based on said first and second ultrasonic signalsand a second speed of sound is computed based on said third and fourthultrasonic signals, the difference in said first and second speeds ofsound providing a level of said stratified flow.
 30. The flow meter ofclaim 27, wherein said first and second ultrasonic signals define agenerally horizontal chord and said third ultrasonic signal defines agenerally vertical chord.
 31. A flow meter suitable to determine anamount of stratified flow through a pipeline, comprising: means forgenerating a first set of signals through said pipeline; means forgenerating a second set of signals through said pipeline, said secondset of signals reflecting from a stratified flow of fluid if any; andmeans for computing said amount of stratified flow based upondifferences in times of flight between said first set of signals andsaid second set of signals.